Three proteins are described which bind androgen with high affinity, low capacity and which demonstrate androgen specificity. One of these (androgen binding protein, ABP) is produced in the Sertoli cell of the testis, secreted into the lumina of the seminiferous tubules, and transported via the efferent ducts to the epididymis. We have purified ABP to homogeneity and are in the process of preparing a radioimmunoassay for its detection. Another binding protein, androgen cytoplasmic receptor (CR) has been reported in a number of andorgen target organs, including the Sertoli cell of the testi and female uterus. Its physiochemical properties are quite different from ABP, but its affinity for DHT is similar to that of ABP. Androgen receptors have never been purified. Sex steroid binding globulin (SBG) is another protein which binds DHT with high affinity. This protein is found in serum and has many physicochemical characteristics which are similar to ABP. A basic knowledge of the active binding sites of these three proteins is important in understanding the dynamic equilibrium of androgens both during transport and at the site of action of these steroid hormones. In this proposal we plan to isolate and purify both CR and SBG to apparent homogeneity, fully characterize each protein with respect to its physicochemical properties, and raise monospecific antibodies to each protein. The binding properties of CR, SBG and ABP will be assessed, including steroid specificity, kinetics and thermodynamics of binding and peptide mapping the binding site. Monospecific antibodies to CR, ABP, and SBG will be used to study the hormonal regulation of the synthesis and secretion of each protein.